Fractal characteristics of coal reservoir micropore system, east margin of ordos basin

摘要

Based on the nitrogen adsorption isotherm and Frenkel-Halsey-Hill (FHH) model, fractal surface characteristics of 56 coals from 14 mine areas in east margin of Ordos Basin are recognized, and the correlation with their physical-chemical property were investigated. The results show that the surface fractal dimension (DS) ranges from 2.02 to 2.91.Two correct DS, which basically equal to each other, can be obtained from the linear segments in which van der Waals force or capillary cohesion dominates the adsorption process. However, the DS calculated from the curve segments, which is in the transitional stage of van der Waals force and capillary cohesion, are usually bigger than the actual values. DS is able to better characterize the micropores structure of coal, showing positive correlation with BET specific surface area and negative correlation with the middle pore diameter. DS are influenced by metamorphic grade and maceral, decreasing with the increase of Carbon, hydrogen content, and the decrease of ash content respectively, and DS are basically greater than 2.5 when vitrinite content (V)≤50%, less than 2.6 when V≥50%.